1. Field of the Invention
The present invention relates to a signal transmission device that transmits an input signal from a pair of input terminals to a pair of output terminals that are electrically insulated from the pair of input terminals. Specifically, the present invention relates to a signal transmission device that can reduce the influence of an external noise magnetic field during signal transmission.
2. Description of the Related Art
Signal transmission devices for transmitting a signal from a pair of input terminals to a pair of output terminals that are electrically insulated from the input terminals are known. One of such signal transmission devices is a device for transmitting a signal using a coil and a magnetic sensor which is insulated from the coil electrically but joined to the coil magnetically.
A representative example of such a device is a transformer. A transformer has an input side coil that is connected to a pair of input terminals and an output side coil that is connected to a pair of output terminals. The input side coil and the output side coil are electrically insulated from each other but magnetically joined with each other. Through electromagnetic induction of the transformer, a signal that is inputted from the pair of input terminals to the input side coil can be transmitted to the pair of output terminals without electrical connection between the pair on input terminals and the output terminals.
There are transformers in which the voltage level of an outputted signal is different from the voltage level of an inputted signal. There are also transformers, such as pulsing transformers, in which the voltage level of an outputted signal is same as the voltage level of an inputted signal.
A signal transmission device that transmits signal from the pair of input terminals to the pair of output terminals insulated electrically from the pair of input terminals can also be realized by using an element such as a hall element or a magnetoresistive element for outputting a signal in response to an acting magnetic field, instead of using the output side coil of the transformer.
In the present specification, the section in the signal transmission device that transmits a signal via a magnetic field in a state of electrical insulation is referred to as a “signal transmission section”. A transformer is an example of a signal transmission section. Moreover, in the signal transmission device, the section that outputs a signal in response to an acting magnetic field is generically referred to as a “magnetic sensor”. Examples of magnetic sensors are the output side coils and the hall elements or magnetoresistive elements that can be used in place of the output side coils. The output side coil may be referred as the “detection coil”. A signal that is outputted by the magnetic sensor may be referred to as a “detection signal”. In the signal transmission device, a section that outputs a signal in response to a signal inputted to the pair of input terminals, on the basis of the detection signal outputted by the magnetic sensor, is referred to as a “signal outputting section”.
Even a signal transmission section alone, such as a transformer, can transmit signal from the pair of input terminals to the output terminals that are electrically insulated from the pair of input terminals. However, when the device transmits a signal via a magnetic field, the magnetic sensor detects not only the magnetic field generated by the input side coil but also an external noise magnetic field. An external noise magnetic field is, for example, the magnetic field generated by other equipments or circuits, or the magnetic field caused by the earth magnetism. The detection signal that is outputted by the magnetic sensor of the signal transmission section includes the signal (that should be transmitted) that results from the magnetic field generated by the input side coil, and a signal (that should not be transmitted) that is caused by an external noise magnetic field. In a signal transmission device in which signal is transmitted using a coil and a magnetic sensor that both are magnetically joined with each other but electrically insulated from each other, it is important to reduce noise signals caused by an external noise magnetic field.
In the present specification, “a signal that should be transmitted essentially”, which is included in the detection signal outputted by the magnetic sensor of the signal transmission section, is referred to as “an essential signal” hereinafter. “A signal that should not be transmitted”, which is included in the detection signal and is caused by a noise magnetic field, is referred to as “a noise signal” hereinafter.
Japanese Patent Application Laid-Open No. 2005-51021 discloses an example of a signal transmission device that reduces a noise signal included in a detection signal outputted from an output side coil of a transformer. In this signal transmission device, a dummy coil is arranged in the vicinity of the output side coil of the transformer. In the detection signal that is outputted from the output side coil of the transformer, a noise signal is added to an essential signal. The dummy coil outputs only the noise signal generated by a noise magnetic field. By determining the difference between a detection signal outputted from the output side coil of the transformer and the noise signal outputted by the dummy coil, the noise signal can be reduced from the detection signal outputted from the output side coil of the transformer.